14:20
Session 4A: System Optimization (2)
Chair: Giampaolo Manfrida
14:20
20 mins
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Deterministic global optimization of the design of a geothermal Organic Rankine Cycle
Wolfgang R. Huster, Dominik Bongartz, Alexander Mitsos
Abstract: Numerical optimization has become a widely used tool in the design of energy systems or chemical processes. However, deterministic global optimization methods, which can guarantee that the returned solution is a global optimum within a specified tolerance, are only recently becoming applicable to challenging design problems due to the high computational demand. We introduced a problem formulation for global flowsheet optimization similar to sequential-modular approaches employed in local optimization. This approach operates on a small subset of the model variables (including the degrees of freedom in designing the process), while most model variables and equations are ‘hidden’ from the optimizer in external modules. The relaxations required for providing lower bounds to the Branch-and-Bound (B&B) solver are obtained through the automatic propagation of McCormick relaxations. This approach eliminates the need for providing bounds on all model variables and enables the optimization of process flowsheets even with a very simple B&B solver.
In this contribution, we apply this approach for deterministic global optimization of process flowsheets to an Organic Rankine Cycle (ORC) for power generation from a fixed geothermal heat source, combined with the use of an air-cooled condenser. For this low-temperature heat source, isobutane is chosen as the working fluid, which is modeled using ideal gas and ideal liquid equations of state with temperature-dependent heat capacities. Assuming steady-state operation, the sizing of components and selection of operating conditions are optimized to either maximize net power output or economic factors. Additionally, different structural options, such as the use of a recuperator, are considered. The air cooling of the condenser in the ORC is accounted for in the optimization, as the design optimization is carried out for different ambient temperatures.
The resulting feasible regions of the optimization problem, which are defined by the physical constraints of the real problem, are visualized for different optimization variables, together with contour lines of the chosen objective functions.
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14:40
20 mins
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Thermoeconomic analysis of recuperative sub- and transcritical Organic Rankine Cycle systems
Oyeniyi Oyewunmi, Steven Lecompte, Michel. De Paepe, Christos Markides
Abstract: There is significant interest in the deployment of organic Rankine cycle (ORC) technology for waste-heat recovery and power generation in industrial settings. This study considers ORC systems optimized for maximum power generation using a case study of an exhaust flue-gas stream at a temperature of 380 °C as the heat source, covering over 35 working fluids and also considering the option of featuring a recuperator. Systems based on transcritical cycles are found to deliver higher power outputs than subcritical ones, with optimal evaporation pressures that are 4 – 5 times the critical pressures of refrigerants and light hydrocarbons, and 1 – 2 times those of siloxanes and heavy hydrocarbons. For maximum power production, a recuperator is necessary for ORC systems with constraints imposed on their evaporation and condensation pressures. This includes, for example, limiting the minimum condensation pressure to atmospheric pressure to prevent sub-atmospheric operation of this component, as is the case when employing heavy hydrocarbon and siloxane working fluids. For scenarios where such operating constraints are relaxed, the optimal cycles do not feature a recuperator, with some systems showing more than three times the generated power than with this component, albeit at higher investment costs.
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15:00
20 mins
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Robust optimization of an Organic Rankine Cycle for heavy duty engine waste heat recovery
Elio Antonio Bufi, Sergio Mario Camporeale, Paola Cinnella
Abstract: In recent years, the Organic Rankine Cycle (ORC) technology has received great interest from
the scientific and technical community because of its capability to recover energy from low-
grade heat sources. In some applications, as waste heat recovery (WHR) in automotive field,
ORC plants need to be as compact as possible because of geometrical and weight constraints.
Effective solutions have been proposed by Honda and BMW for passenger cars and
Cummins for long-haul trucks. Even if the performances of recently developed prototypes
of ORC for automotive applications seem to be promising, with reduction of fuel
consumption up to 12% and engine thermal efficiency improvements of 10%, currently no
commercial ORC solutions in the automotive field are available. This is mainly due to the low
robustness of the present design methods techniques when applied to the large range of
operating conditions on typical duty driving cycles and the subsequent low improvements of
engine global performances compared to the economic effort to realize the ORC components.
In this work, a robust optimization approach is proposed in order to overcome these issues for
a real-world application, namely the recovery of the residual energy from the waste gases of a
heavy duty diesel engine. A parametric optimization of the ORC is carried out by taking into
account uncertainties in the cycle input parameters. A typical engine driving duty cycle is
considered and sampled in order to construct a suitable probability distribution describing the
variability of the exhaust gases mass-flow and temperature. Besides, the turbine and pump efficiencies are also considered as uncertain. By considering six working fluids of interest for WHR applications, all the ORC parameters are optimized by means of a genetic algorithm coupled
with an uncertainty quantification method, based on Monte-Carlo simulations, with
the aim to maximise the cycle thermal efficiency while minimising its variance. Constraints
on the minimal efficiency are also explicitly accounted for. The outcomes of this process are
the ORC parameters that assure the best trade-off between high efficiency and a stable
behaviour of the system, as well as confidence intervals on these parameters.
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15:20
20 mins
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The role of pinch analysis for industrial ORC integration
Donald Olsen, Yasmina Abdelouadoud, Peter Liem, Beat Wellig
Abstract: In many industrial companies a significant proportion of their total energy use is for process heat leading to the ever greater importance of improving thermal energy efficiency. A common technique to improve efficiency is through process heat recovery. Pinch analysis is a key method for optimizing industrial heat recovery resulting in greater energy efficiency and profitability. Pinch analysis also shows how energy conversion units such as heat pumps, combined heat and power systems or organic Rankine cycles (ORC) can be optimally integrated into a process.
An ORC converts low temperature heat (e.g. 100-200°C) into electricity. However, the integration of an ORC requires a sound conceptual design to ensure proper integration. Pinch analysis plays a significant role in the development of such concepts as it identifies and quantifies the amount of waste heat as well as allows the determination of the streams most suitable for the ORC. In this regard, the grand composite curve (GCC) is very important for the conceptual design of an ORC to ensure the proper placement when considering the entire process. A properly integrated ORC that uses waste heat as a heat source must operate below the pinch point, i.e. the ORC takes heat from below the pinch point and converts a part of it to electricity and rejects the remaining heat to the environment.
In this paper the importance and application of pinch analysis in addition to process understanding and data extraction is reviewed. Next a methodology is presented for the correct integration of an ORC into industrial processes based on the solid foundation of a pinch analysis. The methodology stresses improving the overall process energetic efficiency first through heat recovery before using (real) waste heat in an ORC. Finally, the PinCH 3.0 software tool developed at the Lucerne University of Applied Sciences and Arts is shown that supports the integration of an ORC within the background process established in the pinch analysis. An industrial case study is presented to illustrate the effectiveness of a meaningful ORC integration.
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15:40
20 mins
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High temperatur ORC systems
Riccardo Vescovo, Emma Spagnoli
Abstract: ORC systems are well known as the best available technology for electricity production from low-temperature heat sources -such as geothermal ones- and continuous technical development is done to optimize performances by means of finding new working fluids and improved design criteria and thermodynamic cycles.
With regard to medium and high temperature heat sources exploitation for electric power production, ORC technology and the more traditional Steam Rankine Cycle solutions are competing in terms of power production, capital expenditures and operating & maintenance costs to become the preferred choice on a case-by-case basis.
Commercially available ORC technology is currently limited to a maximum working fluid operating temperature of about 300 °C, leading to a strong limitation on performances achievable.
Finding new working fluids that could operate at Higher Temperatures and developing the related technical solutions, will enable to improve ORC technology competiveness also in other niches that now are Steam Rankine Cycles territory.
High Temperature ORC technology will lead to new solutions for both power only production and cogeneration, allowing ORC technology to enter market segments traditionally belonging to other technologies (e.g. Steam Rankine Cycles, Otto Cycles, Bryton Cycles, etc.).
The paper will present the technical and industrial development on High Temperature ORC systems, potential market development and a technology comparison to other generation technologies.
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